The present invention relates to a noise reduction circuit for use with an audio signal system of a magnetic recording and playback apparatus which records an audio signal on a recording medium with a desirable signal-to-noise (S/N) ratio and restores high frequency components of a recorded signal with fidelity while reproducing an audio signal with a minimum of noise.
Generally, an audio signal system of a magnetic recording and playback apparatus is furnished with a noise reduction circuit which includes a differential amplifier, voltage control amplifier, a high pass filter and a low pass filter. In the noise reduction circuit, noise which is attributable to the voltage control amplifier itself appears in an output of the circuit and, hence, the noise of the voltage control amplifier is the S/N ratio determining factor for the whole circuit. In a recording mode of operation, an audio signal accompanying noise attributable to the voltage control amplifier has the level of its high frequency component inclusive of the noise raised by a preemphasis circuit, which is connected to an output of the noise reduction circuit, resulting in magnification of the noise. It is difficult, therefore, to record an audio signal keeping this kind of noise down on a recording medium.
Further, there is another problem associated with a noise reduction circuit having an encoder frequency characteristic which remains flat even in a high frequency range. This brings about an excessive boost of high frequency signal components in the subsequent preemphasis circuit. In turn, the excessively boosted high frequency components are limited in the subsequent limiter and, thus, the original high frequency components are no longer restored in the playback mode of operation in the complementary manner. The result is a deteriorated audio signal out from the noise reduction circuit.